1. Field of the Invention
The present invention relates to an electrophotographic method and an electrophotographic apparatus, and more particularly to an electrophotographic method and an electrophotographic apparatus suitable for a digital electrophotographic method employing a laser beam printer, a digital copying apparatus or the like and an electrophotographic apparatus therefor.
2. Related Background Art
Electrophotographic apparatus such as laser beam printer or copying apparatus is recently attracting attention of the market because of various features such as high image quality and high printout speed. Also outputs of photographs are rapidly increasing in addition to those of characters, thereby increasing the demand for higher image quality of the electrophotographic apparatus. The photosensitive members employed in such electrophotographic apparatus can be classified into organic type photographic members and inorganic type photographic members.
Organic materials as photoconductive materials employed in the electrophotographic photosensitive members have been actively developed in recent years. Particularly, the function-separated photosensitive member composed of a charge generating layer and a charge transporting layer mutually stacked is already commercialized and is adopted, for example, in the copying apparatus and the laser beam printers.
These photosensitive members have generally been evaluated on the durability. The durability can be divided into physical durability relating to the electrophotographic process such as sensitivity, retentive potential, charging ability, faint image and the mechanical durability such as abrasion or scratch on the surface of the photosensitive member resulting from friction, both being important factors determining the service life of the photosensitive member. Among these, it is already known that the defect on the electrophotographic physical durability, particularly the faint image, results from degradation of the charge transporting substance contained in the surface layer of the photosensitive member, by active substances such as ozone or NO.sub.x generated from the corona charger. It is also known that the defect on the mechanical durability is caused by physical contact and friction of paper, cleaning member such as blade or roller, or toner with the photosensitive layer. In order to improve the electrophotographic physical durability, it is important to employ a charge transporting substance which is not easily degraded by the active substance such as ozone or NO.sub.x, and, for this purpose, there is already known to use the charge transporting substance of a high oxidation potential. Also, for improving the mechanical durability, it is important to increase the surface lubricating property to thereby reduce the friction in order to withstand the abrasion by the paper or cleaning member, and to improve the releasing property of the surface in order to prevent filming-melting-adhesion of the toner, and, for this purpose it is already known to mix a lubricant such as fluorinated resin power, fluorinated graphite or polyolefin resin power in the surface layer. However, when the abrasion is significantly lowered, the hygroscopic substances generated by the active substances such as ozone or NO.sub.x are deposited on the surface of the photosensitive member, thereby reducing the surface resistance to induce the lateral movement of the surface charge and result in the faint image (smeared image). Also, since certain abrasion is necessary because of the above-mentioned reason, the potential characteristics such as sensitivity or charging ability inevitably vary in a prolonged period, and the change in the surfacial topography resulting from the abrasion induces light scattering, thereby degrading the image quality.
On the other hand, an example of the inorganic materials employed for the photosensitive member is amorphous silicon (hereinafter, referred to as "a-Si"). In the electrophotographic photosensitive member, the photoconductive material constituting the photosensitive layer is required (1) to have a high sensitivity, a high S/N ratio [photocurrent (Ip)/dark current (Id)] and an absorption spectrum matching the spectral characteristics of the irradiating electromagnetic wave; (2) to have a fast light response and a desired dark resistance; and (3) to be harmless to the human body at the use. The above-mentioned ecological safety at the use is particularly important in case of the photosensitive member to be incorporated into the image forming apparatus for use as an office equipment.
A material meeting these requirements is hydrogenated amorphous silicon (hereinafter, referred to as "a-Si:H"), and application of a-Si:H for the photosensitive member in the image forming apparatus is disclosed, for example, in U.S. Pat. No. 4,265,991. In comparison with the aforementioned organic type photosensitive members, the a-Si:H photosensitive member is provided with various advantages such as (1) very high linearity in the photosensitive characteristics, (2) uniformity in material structure and absence of light scattering, and (3) a high dielectric constant and a strong electric field effect, and is particularly suitable in realizing the high image quality.
Also, U.S. Pat. No. 5,382,487 discloses a photosensitive member, for use in the image forming apparatus, composed of a conductive substrate and a photoconductive layer consisting of a-Si containing halogen atoms (X) as a constituent (hereinafter, referred to as "a-Si:H"). The above-mentioned patent teaches that a heat-resistant photoconductive layer having satisfactory electrical and optical characteristics for use in the photosensitive member for the image forming apparatus can be obtained by adding 1 to 40 atomic % of halogen atoms to a-Si.
Also, in order to improve the electrical, optical and photoconductive characteristics such as dark resistance, photosensitivity and optical response and the environmental characteristics such as moisture resistance in the photoconductive member having a photoconductive layer composed of a deposited a-Si layer, Japanese Patent Application Laid-Open No. 57-115556 discloses a technology of forming a surface layer composed of a non-photoconductive amorphous material containing silicon atoms and carbon atoms on a photoconductive layer composed of an amorphous material containing silicon atoms as a matrix.
Furthermore, Japanese Patent Application Laid-Open No. 60-67951 discloses a technology of stacking a translucent insulating overcoat layer containing amorphous silicon, carbon, oxygen and fluorine on a photosensitive member, and U.S. Pat. No. 4,788,120 discloses a technology of employing in the surface layer an amorphous material containing silicon atoms, carbon atoms and 41 to 70 atomic % of hydrogen atoms as constituents.
Furthermore, Japanese Patent Application Laid-Open No. 57-158650 discloses that a photosensitive member of a high sensitivity and a high resistance for use in the image forming apparatus is obtained by employing a photoconductive layer having a-Si:H containing 10 to 40 atomic % of hydrogen and a ratio of 0.2 to 1.7 in the absorption coefficients of the infrared absorption peaks of 2100 cm.sup.-1 and 2000 cm.sup.-1.
On the other hand, Japanese Patent Application Laid-Open No. 60-95551 discloses a technology, for improving the image quality of the amorphous silicon photosensitive member, of executing the image forming steps of charging, exposure and development and transfer while maintaining the temperature in the vicinity of the surface of the photosensitive member at 30.degree. C. to 40.degree. C., thereby preventing the decrease in the surface resistance caused by the moisture absorption on the surface of the photosensitive member and the image smear (high humidity smear) resulting therefrom.
These technologies have improved the electrical, optical and photoconductive characteristics of the photosensitive member for the image forming apparatus and the environmental characteristics thereof, thereby resulting in the improvement in the quality of the formed image.
On the other hand, in the electrophotographic apparatus, there are generally known the digital system and the analog system. In most of the digital electrophotographic systems, a light emitting member (for example, an LED or a semiconductor laser) is turned on and off according to the image signal, and the emitted light is projected onto the photosensitive member. Consequently, the resulting digital latent image is formed with a group of dots (pixel units), and the solid portion, halftone portion and light portion of the image are represented by varying the density of the dots.
The image forming method in the digital electrophotographic system can be divided into two according to the relation between the image information and the exposure unit, one being an image area exposure (hereinafter, referred to as "IAE") method for exposing the image area, the other being a background area exposure (hereinafter, referred to as "BAE") method for exposing the non-image (background) area.
In the BAE method, in order to develop the non-exposed area where the charge remains, there is employed a developer having the same polarity as the charging polarity of the photosensitive member. Since this relationship is the same as in the electrophotographic apparatus of analog system, the BAE method has the advantages of utilizing the developing mechanism, cleaning mechanism, developer and the like common to the electrophotographic apparatus of analog system. On the other hand, in the IAE method, in order to develop the area where the charge is dissipated by exposure, there has to be executed reversal development with the developer of a polarity opposite to the charging polarity of the photosensitive member. Although these two methods are both commercialized, the selection of these methods is often determined by limitation on the photosensitive member and the developer to be employed.
In the above-described digital electrophotographic systems, there may result distortion of dot shape and scattering of developer in the steps of developing, fixing and the like, whereby the satisfactory gradation property of the output image may not be obtained corresponding to the dot density ratio of the digital latent image.
Also, in case of increasing the resolution by reducing the dot size in order to improve the image quality, the development becomes difficult in the pixels of a low density, while in case of the pixels of a high density a sufficient contrast cannot be obtained between pixels because of the increased influence between the adjacent pixels, so that the gradation property of an image tends to become insufficient particularly in the halftone area.